1. Field of the Invention
The present invention relates to a sensor circuit for a telephone line. More specifically, the present invention relates to a sensor circuit for a telephone line which is provided in a network control unit (NCU).
2. Description of the Prior Art
In order to ring a bell of a telephone in response to a reception of a calling signal of 12-80 Hz, for example in an ON state of a hook switch, an NCU is provided with a calling signal detection circuit. In addition, for detecting an ON/OFF state of the hook switch of the telephone, and etc., the NCU is further provided with a line current supervisor circuit.
In such a calling signal detection circuit and the line current supervisor circuit, as well-known, it is necessary to isolate the circuits from the telephone line. Therefore, in a conventional sensor circuit, by using a photocoupler as a main component, it was intended to ensure the insulation.
For example, as a calling signal detection circuit, there was a circuit in which a full-wave rectifying circuit is connected to the telephone line and a photocoupler is connected to an output of the full-wave rectifying circuit. In addition, as a line current supervisor circuit, there was a circuit in which a pair of photocouplers are inserted in the telephone line such that outputs according to a flowing direction of the line current can be obtained from the photocouplers.
In such a case, because respective circuits are constructed separately from each other, a component for insulation such as a photocoupler is required for each circuit, and therefore, the number of components was large, and thus, assembling of the circuits was troublesome.